The current mirror circuit configuration has found widespread use as an integrated circuit design technique and its operation is well-known to practitioners in the art. In a canonical form as depicted in FIG. 1, the current mirror includes a current drive transistor, shown as Q1 in FIG. 1, whose current I1 is externally fixed or forced in some manner, for example, through the use of a constant current source. The base of Q1 is then attached to a string of controlled current sources Q2, . . . , Qn. If transistors Q1, . . . , Qn are fabricated from an integral piece of semiconductor material, then their Vbe and emitter current characteristics will match. And, if R1=R2= . . . =Rn and if the base-emitter areas are of the same size, then I1=I2= . . . =In. It is also understood that the emitter resistors and emitter areas may be "ratioed" so that the controlled currents, I2, . . . , In, may be set at a predetermined fixed multiple or fixed fraction of I1.
However, in any event it may be seen that I1 contains a current component attributable to the sum of the base currents of I2, . . . , In. If the string of dependent current sources is long (n large) or if the beta's of the transistors are low, as would likely be the case were these devices laterally diffused transistors, then the base drive component of I1 will become large. In this case the assumption I1=I2= . . . =In is no longer valid and the current delivered by the controlled current sources will deviate from the predetermined predicted current.
The thrust of the subject invention is a technique for eliminating this error. With reference to FIG. 1, the technique can be understood as a departure according to which I2 is compared in a feedback loop to the predetermined intended current. Deviations in the value of I2 from the intended value cause an error signal to be developed. The error signal is then used to adjust the drive to Q1 so that the value of I2, as well as the values of the other current sources in the string, are forced back toward the intended value.